1. Field of the Invention
This invention relates generally to microfabricated devices, and more particularly to devices fabricated using silicon oxide or similar material as a sacrificial layer. This invention improves release of mechanical devices formed on the same substrate as integrated circuitry using conventional microfabrication techniques.
2. Description of the Related Art
MicroElectroMechanical Systems (MEMS) combine mechanical structures and microelectronic circuits to create integrated devices. MEMS have many useful applications such as microsensors and microactuators. Examples of microsensors include inertial instruments such as accelerometers and gyroscopes, detectors for gasses such as carbon-monoxide, and pressure sensors. Examples of microactuators include optical mirrors used for video displays, optical switches, or multiplexors, and disk-drive head actuators used for increasing track density.
Many MEMS fabrication methods and techniques are known to those skilled in the art and are documented in publications such as Education in Microelectronics and MEMS by Payne, et al., the IEEE Journal of MicroElectroMechanical Systems, and numerous national and international conferences including: IEEE Solid-State Sensor and Actuator Workshop, IEEE International Conference on Solid-State Sensors and Actuators, and IEEE International Conference on Micro Electro Mechanical Systems. Many of the mechanical elements in MEMS devices are formed using layers of structural and sacrificial materials. A layer of sacrificial material typically supports structural materials during manufacture. The sacrificial layers are generally removed near the end of the manufacturing process to release the mechanical structures from surrounding materials. This is accomplished using a release etch which is typically isotropic and highly selective to the sacrificial material, leaving the structural material largely unaffected. Often a long release etch is required to undercut structural materials for a distance many times greater than the thickness of the sacrificial material. In many cases, etch holes are included in the structural material in an attempt to minimize the required undercutting and thereby shorten the release etch. Minimization of required undercutting can result in closely spaced etch holes that significantly constrain the mechanical design of a structure and reduce the performance of the MEMS device.
In many cases, MEMS sacrificial materials such as silicon oxides are also used in integrated circuits. When circuits are integrated with structures that require a release etch, the integrated circuits must be protected from the release etch. When silicon oxides are used as a sacrificial material, hydrofluoric acid based solutions or vapors may be used as the release etchant. Hydrofluoric based release etches include hydrogen fluoride vapor, hydrofluoric acid, buffered hydrofluoric acid, and various pad etchants. These release etchants can damage unprotected integrated circuitry, particularly when significant undercut is required.
Depositing a release mask may provide protection for integrated circuits and other sensitive elements. The release mask must not only be acid resistant but also impermeable. Since integrated circuits typically include aluminum interconnect, the release mask must also be deposited and removed at temperatures less than 450xc2x0 C. using compatible materials. A variety of organic coatings have been employed as release masks but have proven inadequate for heavily undercutting release etches. Films deposited using chemical vapor deposition (CVD), low pressure CVD (LPCVD), plasma enhanced CVD (PECVD) have also been considered as a release mask. For example, amorphous silicon can be deposited over aluminum metallization using LPCVD or PECVD and can be an effective release mask, however, removal of amorphous silicon without damaging other materials can prove problematic. Other PECVD materials may also be deposited at lower temperatures but typically have poor conformality and are typically low-quality materials and therefore unsuitable as a release mask.
An effective release mask may improve performance, cost, manufacturability, and reliability of MEMS devices. A durable release mask may enable more undercutting during release etches thereby increasing the maximum etch-hole spacing, or even eliminating etch holes that constrain structure design. Large, more elaborate structures may also be designed, including MEMS systems comprising structures, electrical interconnection, and circuitry. The release etch may also be performed using a more aggressive etchant saving fabrication time and expense. Furthermore, the performance of devices such as sensors and actuators may benefit from combining mechanical structures with circuits integrated in the same substrate. Such MEMS devices are often improved as the distance diminishes between mechanical elements and the release-etch-sensitive elements. Increased proximity can reduce parasitic resistance and capacitance associated with electrical interconnect. Reduced parasitic resistance and capacitance yields higher electrical performance and hence a better performing MEMS device. Increased proximity of the different MEMS elements also lowers fabrication costs by increasing the number of batch fabricated components per substrate thereby reducing the cost per component.
The invention is directed to a method of fabricating MEMS systems. The method includes: providing a substrate in which integrated circuits and a sacrificial layer have been formed, forming a release mask including germanium, etching exposed sacrificial material, and removing the release mask. The release mask provides protection for materials that may be adversely affected by the release etch. The mask may be removed without affecting other materials. This invention may potentially be used with a variety of MEMS processes, see for example: U.S. PPA Serial No. 60/127,973, Filed Apr. 6, 1999; U.S. patent application Ser. No. 09/322,381, filed May 28, 1999; Montague et al., U.S. Pat. No. 5,798,283 issued Aug. 25, 1998; Kung, et al., U.S. Pat. No. 5,504,026 issued Apr. 2, 1996; Sherman, et al., U.S. Pat. No. 5,847,280 issued Dec. 8, 1998; Tsang et al., U.S. Pat. No. 5,326,726, issued Jul. 5, 1994; Spangler et al., U.S. Pat. No. 5,343,064, issued Aug. 30, 1994; Bashir et al., U.S. Pat. No. 5,747,353, issued May 5, 1998; Zhang et al, U.S. Pat. No. 5,506,175 issued Apr. 9, 1996; Diem et al., U.S. Pat. No. 5,576,250, issued Nov. 19, 1996.
Implementations of the invention may include the following. Circuit elements may be formed in the substrate including active devices such as transistors. Electrical interconnection may be formed among circuit elements and structural elements. Electrical circuits and electrical interconnection may involve materials that may be adversely affected by a release etch. The structural elements may comprise materials that are not adversely affected by a release etch. The circuit and/or structural elements may be supported by or connected to sacrificial materials that may be removed by a release etch. A temporary release mask may be deposited and defined to allow a release etch to remove sacrificial materials where desirable while leaving other materials undisturbed. The temporary release mask may be removed using an etch process that will not damage materials used in the MEMS device including electrical circuits, interconnection, and structural elements. Sacrificial materials may include silicon oxides either grown on silicon or deposited on the substrate using CVD, LPCVD, or PECVD. Structural materials may include, for example, single crystal silicon, polycrystalline silicon (polysilicon), or silicon nitride deposited using CVD, LPCVD, or PECVD. The release etch may be performed using any of many known silicon-oxide etching chemistries including hydrofluoric acid (HF) or buffered hydrofluoric acid (BHF or BOE). The temporary release mask may be an amorphous or a polycrystalline germanium/silicon mixture deposited using CVD, LPCVD, or PECVD. When the release mask is largely germanium, the temporary release mask removal may be accomplished with an oxidizing etch including water, a hydrogen peroxide solution, or an oxygen plasma.
Advantages of the invention include the following. Performance of devices fabricated in accordance with the invention are improved due to the proximity of interface circuitry built into the same substrate as the microstructures. Proximity of circuitry and microstructures also reduces the cost of manufacture. Etch hole spacing can be increased, providing added flexibility of mechanical structure design. When etch holes can be fully eliminated, depending on the particular MEMS technology, mechanical devices may be suspended from a limited number of locations thereby reducing the effect of substrate stress from, for example, forces applied to a package. In addition to released mechanical structures, the invention also provides a means for forming circuits such as a bandgap reference as a released MEMS element. Addition of a robust temporary release mask allows for greater undercut, more aggressive release-etch chemistries, more robust manufacturing methods, and higher yields. Furthermore, the invention is compatible with existing microfabrication techniques and is compatible with established integrated circuit fabrication processes. Additional benefits are derived from the use of germanium as the temporary release mask. Germanium can be impermeable to and is not attacked by many release etchants. Germanium may be deposited conformally at temperatures low enough to ensure that circuit components such as transistors and metallization are largely unaffected. Conformal deposition ensures the release mask is effective over topology on which the mask is deposited. Germanium is also easily removed without causing damage to materials commonly found in semiconductor processing making germanium an ideal release mask for integrated MEMS.